1. Field of the Invention
The present invention relates to a traction control for a vehicle which controls the torque of the drive wheels of a vehicle by controlling the throttle in order to match the slip rate of the drive wheels with a target slip rate in order to prevent excessive torque in the drive wheels due to an excessive slip rate. The present invention also relates to a method of controlling traction in a vehicle.
2. Description of the Prior Art
When the slip rate of the drive wheels of a vehicle becomes excessive due to excessive torque in the drive wheels, sufficient driving force in the vehicle cannot be achieved, the ability to accelerate deteriorates, the cornering ability of the vehicle deteriorates, and skidding occurs.
Prior art traction controls are known in which, in order to prevent excessive slip caused by excessive torque in the drive wheels of the vehicle, the torque of the drive wheels is controlled by calculating an actual slip rate, and matching the actual slip rate to a specially-determined, prescribed, target slip rate by throttle control or brake control.
Moreover, it is known in the art to carry out traction control by determining whether or not the driving surface is a rough one with much unevenness such as a gravel road, etc., and then, if the road is rough, effecting the most suitable control by appropriately differing the control elements from the traction control on a usual (not rough) road.
A rough road detection device used in a traction control, etc., is disclosed in Japanese Unexamined Patent Publication No. Sho 64-29636, laid open to the public in 1989.
The above-mentioned throttle control for controlling drive wheel torque is based on the value of the drive wheel slip rate which in turn depends on the usual vehicle wheel speed (the rotational speed of the wheels).
However, the wheel speed changes according to the unevenness of the road surface of a rough road, and the above-mentioned slip rate also fluctuates and the slip rate signal contains noise. As a result, a slip rate which is not that great is erroneously determined to be increasing and the throttle is closed in order to reduce the drive wheel torque.
Nevertheless, because the driving resistance is great on rough roads, when the throttle is closed--even though the slip rate is not that high--due to the erroneous determination of slip rate, the torque of the drive wheels is thus decreased so that the vehicle tends to lose speed because of the undue decrease in driving force. Even if the throttle is subsequently opened, the recovery from the loss in speed is delayed.
As a consequence, it is desirable, when driving on rough roads, to prevent the above loss in speed due to the closing of the throttle caused by fluctuations in the speed of the wheels, or to come up with a measure for quickly recovering from such a loss in speed.